1. Field of the Invention
This invention relates generally to new and improved devices for detecting electrostatic discharge (ESD) events occurring in electronic components and electronic assemblies and, more particularly, to reusable detectors which can be positioned directly on or closely adjacent to miniature electronic components and printed circuit board assemblies to detect the occurrence, polarity and approximate magnitude of an ESD event. The invention also relates to devices to protect electronic components and circuits from static discharge, in that the invention can be connected to an electrostatic discharge sensitive device so that ESD events above a predetermined magnitude would result in the destruction of the apparatus of the invention, rather than the ESD sensitive device.
2. Description of the Related Art
Static charges may accumulate on the surfaces of non-grounded conductors and non-conductive surfaces such as most plastics and textiles. These charges will usually remain on the surfaces because there is no path to ground. When a non-grounded conductor comes close to a grounded plane, a spark of high voltage, and potentially destructive low current, will xe2x80x9cleapxe2x80x9d from a point on the non-grounded conductor to the grounded object, causing an electrostatic discharge (ESD) event. When a charged non-conductor comes close to a conductive object, the charge can be induced onto the conductor, which can then rapidly discharge to other conductors.
Electrostatic discharge occurs in many industrial situations, such as manufacturing and assembly processes, electronic testing processes and the like. For example, in the manufacture of semiconductor devices, electrostatic charges may build up and become discharged during various human or machine handling operations wherein semiconductor wafers are processed, tested and packaged. The amount of electrostatic charge accumulated and discharged during handling of work pieces can be sufficiently high to cause a significant number of component failures, reducing the yield of the various manufacturing, testing and packaging operations and thereby increasing the overall cost of the device.
Static electricity can create a wide variety of problems for electronic manufacturers. An ESD event can cause a rapid electron movement through the microscopic conductive paths within a device and generate a heat spike which can cause damage to the gates or other insulating parts of the electronic device. If the discharge is large enough, a portion of the device will be destroyed and the defect will be found during testing. While high levels of electrostatic discharge will result in the immediate destruction of the device, which can be readily discovered during subsequent testing, low level electrostatic discharge may cause latent damage to the device, which may not be detected during initial testing. This latent damage may later result in reduced performance and/or premature product failure.
At present, efforts have been directed toward prevention of ESD events during manufacturing, since there are few known methods to monitor actual events. Knowing where, how large, and when an event occurs is useful in evaluating ESD induced failures so that appropriate preventive measures can be taken to eliminate the source of the ESD event.
Improvements in the manufacture of semiconductor devices have resulted in devices having vastly increased circuit density, reduced active element size and reduced conductor widths. These improvements have increased the overall performance of the devices, but have simultaneously increased the susceptibility of the devices to damage from electrostatic discharge. As a result, electronic devices are potentially susceptible to damage from discharge events as low as 50 volts. Thus, semiconductive devices in routine manufacture and use today are more susceptible than humans, who normally can not feel an electrostatic discharge of less than approximately 3500 volts.
A variety of instruments have been designed and developed to measure electrostatic phenomena in semiconductor device assembly areas. Some of these devices are connected directly to the circuit boards, while other instruments have antennas or other sensors that detect electromagnetic radiation resulting from an electrostatic discharge. In general, these instruments suffer from one or more disadvantages that limit their acceptance and use in the electronic industry, and similar industries. These disadvantages include being too large or expensive, difficult to monitor in real time, and non-reusable, or some combination of these drawbacks. These prior art instruments also generally fail to provide sufficient information to assist in the detection of devices that are damaged or destroyed, including information leading to the detection and elimination of the incipient environmental causes of the ESD events.
One prior art device used to detect ESD events utilizes a silicon Field Effect Transistor (FET) which is destroyed in the process. This device is monitored by first removing it from the circuit board and then inserting it into an external reader. This device has inherent disadvantages which include its inability to be reused, the requirement that it be removed from the circuit to be tested, its inability to measure polarity, and its limited range of one ESD sensitivity level per device.
Other prior art detection devices are known which utilize a liquid crystal display as an indicator and has a clip lead which can be connected to the particular position of interest, e.g., input to an ESD sensitive device. This particular device has a built-in antenna which senses the ESD event and includes hardware for mounting and protecting the device while in use. This device also has inherent disadvantages which include the large size of the unit, its low operating/storage temperature range, its ability to detect only one transient voltage, and the high cost in manufacturing the unit. Additional disadvantages include incompatibility with automatic insertion equipment and the inability of the device to measure polarity.
What has been needed, and heretofore unavailable, is a reliable, low cost, rugged, miniature, reusable device for accurately and economically detecting the occurrence, polarity and magnitude of electrostatic discharge events, including relatively low level events, which can also provide protection to semiconductive devices from large electrostatic discharges. Such a device should be capable of memory retention, so that the occurrence of an ESD event can be detected anytime after it happens. The present invention satisfies these and other needs.
The present invention is directed to a reusable, miniaturized magneto-optic device that detects the current of an electrostatic discharge which may occur during the manufacture, handling or use of electrostatic discharge sensitive components and circuit boards. The present invention also may be used to determine the polarity and magnitude of an electrostatic discharge event. A device made in accordance with the present invention may be manually or automatically read, either by removing the device from the environment being monitored or monitoring the device in situ. The present invention can also be used as a protection device which can be connected to an electrostatic discharge sensitive component to protect it from electrostatic discharge events above a predetermined magnitude.
The electrostatic discharge event detector of the present invention employs the magneto-optic Faraday effect to detect electrical transients. The Faraday effect is a scientific principle which causes the plane of polarization of a polarized beam of light passing through a transparent substance exhibiting the Faraday effect to rotate from the plane of polarization of the incident light by an amount proportional to the magnetic field passing through the substance parallel to the optical axis of the beam of light. Magneto-optic materials exhibiting the Faraday effect are electrically addressable and change or xe2x80x9cswitchxe2x80x9d the direction of magnetization of a magnetic material formed into individual elements, or pixels, through electrical conductors or drive lines that establish a magnetic field having a different direction of magnetization to the initial state. When a magnetic field is established having strength equal to or greater than a predetermined value, the reversal of the direction of magnetization, or switching, occurs.
The electrostatic discharge event detector of the present invention includes a conductor and at least one magneto-optic pixel located adjacent to the conductor which has a first magnetic state and which is capable of switching to a second magnetic state in response to a magnetic field having a field strength that exceeds a predetermined field strength at the location of the magneto-optic pixel. This magneto-optic pixel is capable of switching from its first magnetic state to its second magnetic state when an electrostatic discharge induces a current to flow through the conductor which is of sufficient strength to generate a magnetic field around the conductor that exceeds the predetermined field strength. Afterwards, the magneto-optic pixel can be observed to determine whether it has been switched from its first magnetic state to the second magnetic state which would indicate that an electrostatic discharge event has been experienced and detected by the magneto-optic pixel.
In a presently preferred embodiment, the electrostatic discharge event detector of the present invention also may include a second magneto-optic pixel having a first magnetic state and having the capability of switching to a second magnetic state in response to a magnetic field. This second magneto-optic pixel would be located next to the conductor on a side opposite from the first magneto-optic pixel. As a result, the detector would utilize a pair of magneto-optic pixels located on opposite sides of the conductor which can be used to determine the polarity, or direction, of the current flow of the electrostatic discharge event. Such a determination may prove important in analyzing the electrostatic discharge event, as it can indicate on which surface the electrostatic charge was accumulating.
A further embodiment of the present invention provides information about the magnitude of the electrostatic discharge event, provided it exceeds a minimum magnitude. In this particular embodiment, the ESD detector includes a set of pixels which are arranged in an array around a conductor. Each magneto-optic pixel is separated from the conductor by a distance which is different from any other pixel. Since the strength of the magnetic field surrounding the conductor is inversely proportional to the distance from the conductor where the magnetic field strength is measured, the different pixels will have altered magnetic states dependent upon the strength of the magnetic field at each location. Since the strength of the magnetic field is related to the magnitude of the current flowing through the conductor induced by the electrostatic discharge event, by observing which pixels have altered magnetic states, one can determine the voltage of the ESD event. Additionally, the pixels can be placed in pairs on opposite sides of the conductor to determine polarity.
The readout of the electrostatic discharge event detector can be achieved either manually by observing the reflected light through a polarizing microscope or automatically using a version of a pattern recognition instrument. In either case, it is not necessary to physically contact the ESD detector of the present invention. Since an ESD event would form patterns or switched regions depending on the polarity and the voltage of the event, a readout unit could be placed adjacent to the detector, allowing each electro-static discharge event to be recorded. Alternatively, the detector can be observed continuously to record the time and conditions of each ESD event. A record of the static events could then be linked to a computer for statistical process control.
After the detection of an ESD event, the detector of the present invention would need to be reset, i.e., the active region would have to be returned to its initial condition. This can be accomplished with an externally packaged reset instrument which uses external magnetic fields induced by an electro-magnetic coil or permanent magnet to xe2x80x9creswitchxe2x80x9d the magneto-optic pixels of the detector. The operation of the reset function is determined by the same magneto-optic effect that determines the sensitivity of the detector to an ESD event. An electro-magnet of sufficient field strength and uniformity can be used to reset the device by subjecting the detector to a short pulse which resets the magnetic state of each magneto-optic pixel.
The present invention is also directed to a method for detecting the occurrence of electrostatic events utilizing a magneto-optic detector as the ones herein described. Other methods of the present invention are directed to detecting the polarity and the magnitude of the electrostatic discharge event.
The present invention provides a reliable, low cost miniature detector for accurately and economically detecting the occurrence, polarity and magnitude of electrostatic discharge events, including relatively low level events. The present invention is rugged and relatively inexpensive, allowing the device to be utilized with electronic components and circuit board assemblies to detect ESD events. Additionally, the present invention can also be connected to an electrostatic discharge sensitive component to prevent the component from being destroyed in the event of a sufficiently high ESD event. In such an embodiment, the invention is constructed such that the conductor for conducting the ESD charge will be destroyed by an ESD event over a threshold chosen to protect the device being maintained from destruction. However, the protection mode of the present invention is not reusable as it acts as a fuse to protect the ESD sensitive device from possible damage. These and other features and advantages of the invention will become apparent from the following detailed description, when taken in conjunction with the accompanied exemplary drawings.